During the past decade, intelligent agents have been applied in attempts to a number of collaborative engineering design systems. Intelligent interface agents or personal assistants play a very important role in such systems. A fundamental issue in designing a personal assistant is how to build adequate models of the user and the domain. This paper proposes a cognitive user model of a personal assistant in collaborative design environments, which comprises a user interest model, a user behavior model, an inference component and a collaboration component. These models and components can help agents effectively achieve the user's goals. The proposed user model is being implemented in a coordinated, intelligent, rational agent (CIR-Agent) model on a FIPA-compliant platform. A case study in a multidisciplinary design optimization (MDO) software environment is also presented.